Atmospheric pressure ionization and, in particular, the combination of electrospray ionization with ion trap mass analyzers has become an extremely powerful analytical technique for organic and biochemical analyses. This technique was originated at Oak Ridge National Laboratories (G. J. Van Berkel, S. A. McLukey and G. L. Glish, Anal. Chem., v.62, 1284, 1990).
An improved ion sampling interface with pure electrostatic ion optics for the ion trap mass analyzer was developed later and claimed in the U.S. Pat. No. 5,352,892 and the U.S. Pat. No. 5,268,572. Both designs described in these patents comprise oversized vacuum systems and have a relatively low sensitivity for the size of the utilized vacuum pumps.
Lately, it was established that quadruple ion guides can be a very efficient means for ion transportation through the intermediate pressure range (10.sup.-1 -10.sup.-4 Torr) (D. J. Douglas and J. B. French, J.Am.Soc. Mass. Spectrom., 1992, 3, 398-408). The quadruple ion guide as an ion transportation means was used in several designs. For example, G. Whitehouse (the U.S. Pat. No. 5,652,427) describes a mass spectrometer system with a long ion guide penetrating two differentially pumped vacuum chambers for separating ions from neutrals and interfacing the ion trap mass analyzer chamber. To achieve the desirable differential pumping ratio between chambers the length of the ion guide has to be sufficient to provide appropriate attenuation in the gas load. There are several disadvantages associated with this approach including the effect of ion accumulation in the long ion guides. The ion accumulation within the ion guide results in non-linear ion signal response as well as matrix effects (WO 97/07530).
In yet another designs (the U.S. Pat. No. 5,179,278) the ion guide was placed before the ion trap mass analyzer as an accumulative device with a specially designed pulsed ion optics to improve the duty cycle for the ion trap. This system was very complex, and the design of ion optics may produce matrix effects.
LCQ instrument manufactured by Finnigan Inc incorporates two short ion guides separated with a restricting element in between. The differential pumping ratio was improved in this design and no matrix effects were reported, however the direct coupling of the ion guide to the ion trap resulted in decreased sensitivity. In Esquire instrument manufactured by Bruker/Hewlett-Packard pair of short ion guides were used in conjunction with two additional electrostatic lenses just in front of the ion trap mass analyzer. That configuration solves most of the problems associated with previous designs, however, it is more complex, expensive and the differential pumping ratio is not improved substantially compare to the prior art systems.
Accordingly, it is desirable to provide a mass spectrometer system with improved ion transportation from the atmospheric pressure region to the ion trap mass analyzer vacuum region and decreased pumping load.